This invention relates to gas turbines and more particularly to the mounting of a starter/generator (or S/G for “Starter/Generator”) on a transmission housing or an accessory gear box. The field of application of the invention is that of gas turbines for aeronautical engines, of aeroplanes or of helicopters, as well as for auxiliary power units (or APU, for “Auxiliary Power Unit”).
In a gas turbine, a certain number of items of equipment or accessories are driven by a mechanical transmission from mechanical power taken from a turbine shaft. This mechanical transmission, which comprises a set of gear wheels housed in a casing, is called a transmission housing or an accessory gear box. The accessories particularly comprise various different pumps for the production of hydraulic power, the supply of fuel, lubrication and one or more electric S/Gs.
When the gas turbine is in operation, the or each S/G operates as an electric generator and produces voltage which supplies one or more electrical energy distribution centres for the aeroplane or helicopter and its engine or engines.
When the gas turbine is at rest, an S/G can operate as a starter, being supplied by an external energy source in order to start the gas turbine by rotating the turbine shaft to which the transmission housing is connected.
An S/G of a known type comprises a principal synchronous generator with a principal rotor and a principal stator and an energizer having an armature in the rotor and a field winding in the stator. The energizer armature supplies the principal rotor of the synchronous generator through a rectifier bridge with rotating diodes. In electrical energy generation mode, the armature of the synchronous generator produces an AC voltage as a result of the rotational drive of the field winding supplied with a direct current delivered by the energizer bridge of diodes, the frequency of the AC voltage produced being variable in accordance with the speed of rotation. In starting mode, the principal rotor supplied by the energizer and the principal stator supplied with AC voltage from an external source operate like a synchronous motor.
Such a known S/G is a relatively voluminous piece of equipment which is usually fitted in a special casing on one side of the transmission housing and which is mechanically connected to it. In addition to the electrical equipment, the S/G comprises a lubrication system with a lubricant tank and a pump driven by gears. The outcome of this is great bulk and mass, all the more so as several S/Gs can be mounted on one single transmission housing.